This invention relates to cutting elements used in drag bit for drilling earth formations. Specifically this invention relates to cutting elements having a unplanar interface including a non-uniform portion between their substrate and their cutting layer.
A typical cutting element is shown in FIG. 1. The cutting element typically has cylindrical cemented carbide substrate body 2 having an end face or upper surface 3. An ultra hard material layer 4, such as polycrystalline diamond or polycrystalline cubic boron nitride, is bonded on to the upper surface forming a cutting layer. The cutting layer can have a flat or a curved upper surface 5.
The problem with many cutting elements is the development of cracking, spalling, chipping and partial fracturing of the ultra hard material cutting layer at the layer""s region subjected to the highest impact loads during drilling. This region is referred to herein as the xe2x80x9ccritical regionxe2x80x9d. These problems are caused by the generation of peak (high magnitude) stresses imposed on the ultra hard material layer at the critical region during drilling. Because the cutting elements are typically inserted into a drag bit at a rake angle, the critical region includes a portion of the ultra hard material layer near to and including a portion of the layer""s circumferential edge 6.
Another problem facing cutting elements is the delamination and/or the exfoliation of the ultra hard material layer from the substrate of the cutting element resulting in the failure of the cutter. Delamination and/or exfoliation become more prominent as the thickness of the diamond layer increases.
The present invention provides for cutting elements or inserts which are mounted in a bit body. The cutting elements have a body over which if formed an ultra hard material cutting layer. The inventive cutting elements have an increased thickness of the ultra hard material cutting layer at their critical region, i.e., the region of the cutting element subjected to the highest impact loads during drilling. This region is generally defined beginning at the edge of the cutting element which contacts the earth formations during drilling and can span up to 50% of the cross-sectional area of the cutting element. Preferably, the critical region extends to an area between 45xc2x0 and 70xc2x0 on either side of the point of contact of the cutting element with the earth formation and inward to an area near the central axis of the cutting element.
A main depression is formed on the body (i.e., the substrate) end face (i.e., the upper surface) of the cutting element covering the critical region. The main depression is defined by multiple secondary depressions defining a main depression surface having a depth which increases in an outward radial direction along a critical diameter and which decreases arcuately on either side of the critical diameter. The critical diameter is the diameter that intersects the point of contact between the edge of the cutting element and the earth formation. An ultra hard material layer is bonded to the end surface of the substrate and has either a curved or flat upper surface such that an increased thickness of the ultra hard material layer is formed over the critical region with the maximum ultra hard material thickness occurring at or proximate the edge portion or edge point of the cutting element making contact with the earth formations during drilling.
In alternate embodiments, more than one main depressions are formed on the substrate upper surface. In this regard, as the ultra hard material layer wears due to drilling, the cutting element can be detached, rotated and re-attached to the bit body so as to orient another main depression on the critical region of the cutting element. In this regard, the cutting element can be used again.
In one alternate embodiment, a cutting element is formed with two main depressions. In a preferred embodiment, the two main depressions are oriented such that their central longitudinal axis form an angle relative to each other that is less than 180xc2x0 but greater than 170xc2x0.
In another embodiment, two main depression may used such that their central longitudinal axes are oriented at 180xc2x0 to each other. In a further alternate embodiment, three main depressions are formed on the upper surface of the substrate such that their central longitudinal axes are spaced apart and oriented at 120xc2x0 to each other. In further alternate embodiment, four main depressions are formed. Preferably, with this embodiment the longitudinal central axis of the four main depressions are spaced apart and oriented at 90xc2x0 to each other.
Moreover, a circumferential groove is formed on the outer surface of the body of the cutting element and spans an arc that is approximately the same as the arc spanned by the critical region of the cutting element. The groove is preferably symmetric about the critical diameter of the cutting element. An ultra hard material is packed into the groove forming a secondary cutting surface for improving the cutting efficiency of the cutting element as well as delaying the erosion of the cutting element during drilling.